This invention relates to an image processing method and apparatus. More particularly, the invention relates to an image processing method and apparatus applicable to a copier or printer, for example, wherein it is possible to output an image having a resolution higher than that of an input image in at least one direction.
A problem encountered with conventional printers and the like is the occurrence of conspicuous jaggies along the contours of font character images input to the printer. The reason for this is that since a character image is composed of black and white levels, even fine characters exhibit noticeable jaggies owing to contrast.
Processing referred to as "smoothing" generally is used to solve this problem. In a laser printer, such processing makes use of the fact that individual pixels forming the character image can be subdivided into a plurality of fine pixels in the scanning direction of the laser beam and involves detecting a specific pattern that appears on the contour of a character and replacing the specific pattern with fine pixels in the scanning direction, thereby reproducing the character contour smoothly.
However, smoothing is processing premised on bilevel font character images that are stored by computer or calculated and generated by computer. Consequently, when smoothing processing is applied to an image obtained by binarizing, through pseudo-halftone processing such as the error diffusion method, a multilevel gradation image optically read by a scanner or the like, unnatural jaggies occur at the contour of a character in order to express halftones at the boundary between black and white on the character contour. As a result, the contour does not match the pattern that should be obtained with smoothing processing and almost no smoothing is achieved.
A particular problem arises with an image obtained by binarizing a faint character image. Portions of such an image that should be straight lines become broken up or discontinuous, giving rise to a marked decline in character quality. Since such portions do not undergo smoothing, applying smoothing processing does not improve character quality.